It is specified here that, in the whole application, the terms “lower” and “upper,” respectively “below” and “above,” “upstream” and “downstream” should be understood with a printhead oriented downwards, i.e. with the drop generator above electrodes of the head and a direction of inkjet flow (segments or drops) downwards. Thus, the lower end of an electrode designates the end that is on bottom. Likewise, the further downstream electrode of a pair designates the electrode of that pair in last place opposite an inkjet segment formed or an ink drop formed from a nozzle of the printhead.
It is specified that, by convention, an even jet segment and an odd jet segment (with opposite parity) are defined to designate two jet segments respectively coming from two nozzles arranged to be adjacent in the printhead according to the invention.
A printhead for a binary continuous jet printer is described in the application for patent US 20100045753 in the applicant's name. Such a printhead comprises a so-called multi-nozzle generator with a body including one or several ink intake conduits communicating with a plurality of stimulation chambers to pressurize the ink therein. Each stimulation chamber is in communication with an ink discharge nozzle via a conduit. Each stimulation chamber is mechanically coupled with a single actuator. A given actuator is arranged relative to the body so as to cause, by electrical pulse, a stimulation in the stimulation chamber, typically a pressure wave in the volume of ink contained in the stimulation chamber. All of the nozzles are aligned along an alignment axis and arranged in a same plane.
The continuous inkjet printer is also provided with control means able to send electrical pulses to each actuator and detection means able to detect the relative position between the printhead and a printing medium.
During operation, the pressurized ink is discharged from one or several stimulation chambers through the conduit(s) and the corresponding discharge nozzle(s). The ink discharged from each nozzle then forms a jet having a determined speed. At the outlet of the nozzle, and for a short distance, the trajectory of the jet coincides with the longitudinal axis of the nozzle.
Each stimulation of the ink contained in a chamber by the associated actuator causes a break in the jet of ink discharged from the nozzle. A shorter length between two consecutive stimulations causes the formation of drops, while a longer duration causes the formation of jet segments. The jet segments thus formed are deflected from their initial trajectory and recovered by a recovery gutter. The drops, which are not deflected, leave the printhead to impact a printing support. The continuous jet printing technology thus implemented is called binary because there may or may not be deflection, in a binary manner.
The deflection of the jet segments is obtained by deflection electrodes whereof the electrical power causes the appearance of electrical charges on the surface of the jets. The jet portions thus charged, which, after breaking of the jet, will form segments, are attracted towards said electrodes, which deflects them from their initial trajectory. By construction, the deflection electrodes are arranged sufficiently downstream of the discharge nozzles to have no electrostatic influence on the drops formed upstream of said electrodes.
The deflection electrodes are grouped together in pairs, each electrode of a pair being supplied in phase opposition with the other electrode in the pair. It is thus possible to obtain a total electrical charge supported by a jet segment that is zero or weak.
In operation, the printing support moves forward perpendicularly to the alignment axis of the nozzles and its relative position relative to the printhead is detected. At each relative position where it is necessary to perform ink printing, a position cue is sent to the printing control means. Upon receiving that cue, these printing control means send an electrical stimulation pulse to the actuator(s) needing to be stimulated to obtain the desired printing pattern. In other words, each position cue has a corresponding printing of what is called a screen.
The inventors noticed that after a certain operating duration of a binary continuous inkjet printer as described above, ink was dirtying the deflection electrodes to the point of damaging their effectiveness and sometimes causing malfunctions of the printer.
This flaw is remedied by a periodic operation consisting of systematically cleaning the electrodes. This periodic operation does, however, have the major drawback of interrupting printing.
The aim of the invention is then to propose a solution making it possible to increase the printing period of a binary continuous inkjet printer, between two consecutive cleaning operations to clean its printhead.